DESCRIPTION ( Verbatim from the Applicant's Abstract): The long range goal of this work is to clarify the cellular and molecular events that underlie hormonal control of the development of sexually dimorphic neural pathways in the mammalian forebrain. Central to this goal is a clear understanding of how steroid hormones influence the development of connections between subpopulations of neurons that control sex specific behaviors and physiological responses. The proposed project will use the connections of the anteroventral periventricular nucleus of the preoptic region (AVPV) as a model system to study how sex steroid hormones specify sexually dimorphic patterns of afferent and efferent connections. The overall hypothesis is that the higher levels of estradiol present in males acts on the AVPV through the estrogen receptor alpha (ERa) during the first few days of life to differentiate its neurons and promote innervation by neurons of the principal nucleus of the bed nuclei of the stria terminalis (BSTp) while at the same time suppressing formation of projections from the AVPV to neuroendocrine neurons. Axonal labeling and histochemical methods, as well as both in vivo and in vitro experimental models, will be used to test this hypothesis.